Combined implantable ventricular defibrillator and pacemaker stimulation devices are well known in the art. Such devices permit a heart to be paced for treating bradycardia, for example, while also detecting for ventricular fibrillation and ventricular tachycardia and applying defibrillating electrical energy, cardioversion shocks or antitachycardia pacing pulses to the heart when fibrillation or tachycardia is detected.
One problem that must be addressed in such devices is the need to reliably sense R waves to support fibrillation detection. To this end, implantable cardiac devices including defibrillation capability generally include an automatic sensing control. The aim of such control is to maintain the sensitivity setting low enough (sensitive enough) for detecting low amplitude R wave electrical activity of the heart during fibrillation while avoiding over-sensing which could result in a T wave or noise being sensed by the pacemaker and mistaken for an R wave.
In the prior art, automatic sensing control has been performed by first establishing a ventricular refractory period (VREF) upon sensing an R wave and continuing the VREF for a pre-determined time such as 100 to 140 milliseconds. Following the VREF, the sensing threshold is set at an initial level and then decreased thereafter from the initial threshold level to a minimum threshold level where it is held until the next paced or sensed event. The initial threshold, refractory period, threshold decay rate, and minimum threshold are selected so that the threshold is above the amplitude of the T waves or noise when they occur.
These sensing parameters can be initially set toward achieving the desired sensing threshold characteristics. Unfortunately, many automatic sensing controls have parameters optimized for sensing during normal sinus rhythm and not optimized for sensing to support fibrillation detection. These systems might undersense during fibrillation resulting in a fibrillation episode going undetected. To overcome this, once initially set, the initial threshold may be varied as a function of rate and amplitude of a sensed event. More specifically, the initial threshold is decreased with decreased event amplitude and increasing rate which is generally associated with decreased event amplitudes. This would seem to be the correct course of action, to make sensing more sensitive with decreased amplitude and increased rate. However such processes are unstable because of positive feedback. Once there is false sensing, such as in sensing noise, these processes become more sensitive resulting in further sensing of noise. This continued false sensing can result in the false detection of fibrillation.